In order to short anything you have to have margin which is essentially collateral to back the position to reasonably ensure the shares will be returned in the future, if not, you're liquidated. Cash accounts can't be liquidated. Believe it or not, your financial institution can lend your holdings without your permission, so unless you have an open order on your holdings or you have a physical certificate, if approached, a broker will deal your holdings as a niminee account. That's why everyone should put in sell orders tomorrow for all their holdings at 10$.